Balanced positioning system for use in lithographic apparatus

ABSTRACT

A balanced positioning apparatus comprises a balance mass which is supported so as to be moveable in the three degrees of freedom, such as X and Y translations and rotation about the Z-axis. Drive forces in these degrees of freedom act directly between the positioning body and the balance mass. Reaction forces arising from positioning movements result in corresponding movement of the balance mass and all reaction forces are kept within the balanced positioning system. The balance mass may be a rectangular balance frame having the stators of two linear motors forming the uprights of an H-drive mounted on opposite sides. The cross-piece of the H-drive spans the frame and the positioned object is positioned within the central opening of the frame.

[0001] The present invention relates to balanced positioning systems,such as may be used to position a moveable object in at least threedegrees of freedom. More particularly, the invention relates to the useof such a balanced positioning system in lithographic projectionapparatus comprising:

[0002] an illumination system for supplying a projection beam ofradiation;

[0003] a first object table for holding patterning means capable ofpatterning the projection beam according to a desired pattern;

[0004] a second object table for holding a substrate; and

[0005] a projection system for imaging the patterned beam onto a targetportion of the substrate.

[0006] The term “patterning means” should be broadly interpreted asreferring to means that can be used to endow an incoming radiation beamwith a patterned cross-section, corresponding to a pattern that is to becreated in a target portion of the substrate; the term “light valve” hasalso been used in this context. Generally, the said pattern willcorrespond to a particular functional layer in a device being created inthe target portion, such as an integrated circuit or other device (seebelow). Examples of such patterning means include:

[0007] A mask held by said first object table. The concept of a mask iswell known in lithography, and its includes mask types such as binary,alternating phase-shift, and attenuated phase-shift, as well as varioushybrid mask types. Placement of such a mask in the projection beamcauses selective transmission (in the case of a transmissive mask) orreflection (in the case of a reflective mask) of the radiation impingingon the mask, according to the pattern on the mask. The first objecttable ensures that the mask can be held at a desired position in theincoming projection beam, and that it can be moved relative to the beamif so desired.

[0008] A programmable mirror array held by a structure, which isreferred to as first object table. An example of such a device is amatrix-addressable surface having a viscoelastic control layer and areflective surface. The basic principle behind such an apparatus is that(for example) addressed areas of the reflective surface reflect incidentlight as diffracted light, whereas unaddressed areas reflect incidentlight as undiffracted light. Using an appropriate filter, the saidundiffracted light can be filtered out of the reflected beam, leavingonly the diffracted light behind; in this manner, the beam becomespatterned according to the addressing pattern of the matrix-addressablesurface. The required matrix addressing can be performed using suitableelectronic means. More information on such mirror arrays can be gleaned,for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, which areincorporated herein by reference.

[0009] A programmable LCD array, held by a structure which is referredto as first object table. An example of such a construction is given inU.S. Pat. No. 5,229,872, which is incorporated herein by reference.

[0010] For purposes of simplicity, the rest of this text may, at certainlocations, specifically direct itself to examples involving a mask;however, the general principles discussed in such instances should beseen in the broader context of the patterning means as hereabove setforth.

[0011] For the sake of simplicity, the projection system may hereinafterbe referred to as the “lens”; however, this term should be broadlyinterpreted as encompassing various types of projection system,including refractive optics, reflective optics, and catadioptricsystems, for example. The illumination system may also includecomponents operating according to any of these design types fordirecting, shaping or controlling the projection beam of radiation, andsuch components may also be referred to below, collectively orsingularly, as a “lens”. In addition, the first and second object tablemay be referred to as the “mask table” and the “substrate table”,respectively.

[0012] Lithographic projection apparatus can be used, for example, inthe manufacture of integrated circuits (ICs). In such a case, thepatterning means may generate a circuit pattern corresponding to anindividual layer of the IC, and this pattern can be imaged onto a targetportion (comprising one or more dies) on a substrate (silicon wafer)that has been coated with a layer of radiationsensitive material(resist). In general, a single wafer will contain a whole network ofadjacent target portions that are successively irradiated via theprojection system, one at a time. In current apparatus, employingpatterning by a mask on a mask table, a distinction can be made betweentwo different types of machine. In one type of lithographic projectionapparatus, each target portion is irradiated by exposing the entire maskpattern onto the target portion in one go; such an apparatus is commonlyreferred to as a wafer stepper. In an alternative apparatus—commonlyreferred to as a step-and-scan apparatus—each target portion isirradiated by progressively scanning the mask pattern under theprojection beam in a given reference direction (the “scanning”direction) while synchronously scanning the substrate table parallel oranti-parallel to this direction; since, in general, the projectionsystem will have a magnification factor M (generally <1), the speed V atwhich the substrate table is scanned will be a factor M times that atwhich the mask table is scanned. More information with regard tolithographic devices as here described can be gleaned, for example, fromU.S. Pat. No. 6,046,792, incorporated herein by reference.

[0013] In general, apparatus of this type contained a single firstobject (mask) table and a single second object (substrate) table.However, machines are becoming available in which there are at least twoindependently movable substrate tables; see, for example, themulti-stage apparatus described in U.S. Pat. No. 5,969,441 and U.S. Ser.No. 09/180,011, filed Feb. 27, 1998 (WO 98/40791), incorporated hereinby reference. The basic operating principle behind such a multi-stageapparatus is that, while a first substrate table is underneath theprojection system so as to allow exposure of a first substrate locatedon that table, a second substrate table can run to a loading position,discharge an exposed substrate, pick up a new substrate, perform someinitial metrology steps on the new substrate, and then stand by totransfer this new substrate to the exposure position underneath theprojection system as soon as exposure of the first substrate iscompleted, whence the cycle repeats itself; in this manner, it ispossible to achieve a substantially increased machine throughout, whichin turn improves the cost of ownership of the machine.

[0014] In a known lithographic apparatus, the drive unit of thepositioning device for the substrate table comprises two linear Y-motorseach of which comprises a stator extending parallel to the Y-directionand secured to a base of the positioning device, and a translator(Y-slider) movable along the stator. The base is secured to the frame ofthe lithographic device. The drive unit further comprises a linearX-motor that comprises a stator extending parallel to the X-directionand a translator (X-slider) which can be moved along the stator. Thestator of the X-motor is mounted on an X-beam that is secured, near itsrespective ends, to the translators (Y-sliders) of the linear Y-motors.The arrangement is therefore H-shaped, with the two Y-motors forming theuprights and the X-motor forming the cross-piece, and this arrangementis often referred to as an H-drive.

[0015] The driven object, in this case the substrate table, can beprovided with a so-called air foot. The air foot comprises a gas bearingby means of which the substrate table is guided so as to be movable overa guide surface of the base extending at right angles to theZ-direction.

[0016] In a lithographic apparatus, reactions on the machine frame toacceleration forces used to position the mask (reticle) and substrate(wafer) to nanometer accuracies are a major cause of vibration,impairing the accuracy of the apparatus. To minimize the effects ofvibrations it is possible to provide an isolated metrology frame, onwhich all position sensing devices are mounted, and to channel allreaction forces to a so-called force or reaction frame that is separatedfrom the remainder of the apparatus.

[0017] In an alternative arrangement, the reaction to the driving forceis channeled to a balance mass, which is normally heavier than thedriven mass which is free to move relative to the remainder of theapparatus. The reaction force is spent in accelerating the balance massand does not significantly affect the remainder of the apparatus.Balance masses moveable in three degrees of freedom in a plane aredescribed in WO 98/40791 and WO 98/28665 (mentioned above), as well asU.S. Pat. No. 5,815,246.

[0018] EP-A-0,557,100 describes a system which relies on activelydriving two masses in opposite directions so that the reaction forcesare equal and opposite and so cancel out. The system described operatesin two dimensions but the active positioning of the balance massnecessitates a second positioning system of equal quality and capabilityto that driving the primary object.

[0019] An object of the present invention is to provide a balancingsystem that is readily extendable to multiple degrees of freedom and isusable with various different drive mechanisms.

[0020] According to the present invention there is provided alithographic projection apparatus comprising:

[0021] an illumination system for supplying a projection beam ofradiation;

[0022] a first object table for holding patterning means capable ofpatterning the projection beam according to a desired pattern;

[0023] a second object table for holding a substrate; and

[0024] a projection system for imaging the patterned beam onto a targetportion of the substrate; and

[0025] a balanced positioning system capable of positioning at least oneof said object tables in more than three degrees of freedom, thepositioning system comprising:

[0026] at least one balance mass;

[0027] bearing means for movably supporting said balance mass;

[0028] coarse positioning means for positioning said object table infirst to third degrees of freedom, said three degrees of freedom beingtranslation in first and second directions and rotation about a thirddirection, said first, second and third directions being substantiallymutually orthogonal; and

[0029] fine positioning means for positioning said object table in atleast a fourth degree of freedom substantially orthogonal to said first,second and third degrees of freedom, said coarse and fine positioningmeans being arranged so that reaction forces from said coarse and finepositioning means are channeled to said balance mass;

[0030] characterized in that:

[0031] said balance mass is supported by said bearing means so as to besubstantially free to move in at least said fourth degree of freedom.

[0032] The long stroke (coarse) positioning system of a lithographyapparatus is normally arranged to position the apparatus in X, Y and Rzdegrees of freedom whilst a short stroke (fine) positioning systemprovides higher-precision positioning over all 6 degrees of freedom(i.e. X, Y, Z, Rz, Ry, and Rx). The positioning movements of the shortstroke positioning system can be a source of undesirable vibrations inthe apparatus. These movements are often of much higher frequency thanmovements of the long stroke positioning system and can involve highaccelerations so that the reaction forces are large, even though themoving mass is smaller. By arranging for the reaction forces of the finepositioning means to be channeled to the balance mass, which is free tomove in at least one additional degree of freedom, directly or via thecoarse positioning means, the present invention ensures that allreaction forces are confined to the balanced positioning system andvibrations in the remainder of the apparatus are minimized.

[0033] The balance mass may be a single body moveable in at least fourdegrees of freedom or may be made up of several parts separatelymoveable in one or more degrees of freedom. For example, in anembodiment of the invention a first part of the balance mass is a framemoveable in the first to third degrees of freedom (e.g. X, Y and R_(z))and surrounding the object table whilst a second part of the balancemass is disposed underneath the object table and is moveable in at leastthe fourth degree of freedom (e.g. Z).

[0034] According to a further aspect of the present invention there isprovided a lithographic projection apparatus comprising:

[0035] an illumination system for supplying a projection beam ofradiation;

[0036] a first object table for holding patterning means capable ofpatterning the projection beam according to a desired pattern;

[0037] a second object table for holding a substrate; and

[0038] a projection system for imaging the patterned beam onto a targetportion of the substrate; and

[0039] a balanced positioning system capable of positioning at least onof said object tables in at least three degrees of freedom, thepositioning system comprising:

[0040] at least one balance mass;

[0041] bearing means for supporting said balance mass so as to besubstantially free to move in said three degrees of freedom; and

[0042] driving means for acting directly between said object table andsaid balance mass to position said object table in said three degrees offreedom; characterized in that:

[0043] said balance mass comprises a generally rectangular frame havingits sides generally parallel to said first and second directions, and acentral opening in which said object table is at least partly disposed.

[0044] With the balance mass in the form of a rectangular frame, thedrives forming the uprights of a so-called H-drive arrangement caneasily be integrated into the sides of the frame ensuring that thereaction forces all act directly between balance mass and driven objecttable. Also, because the driven object table sits within the centralopening of the balance frame the distance in the Z-direction between thecenters of gravity of the balance frame and the driven mass is reduced.

[0045] To reduce the excursions of the balance mass, and hence theoverall footprint of the apparatus, it is preferred that the balancemass is considerably more massive, preferably at least five times, thanthe positioned object. In this regard, all masses that move with thebalance mass are considered part of it and all masses that move with thepositioned object are considered part of that.

[0046] It should be noted that in embodiments of the invention accordingto either of the aspects described above, multiple object (mask orsubstrate) tables may be provided and the reaction forces to the driveforces of two or more tables may be directed to a common balance mass ormasses.

[0047] According to yet a further aspect of the present invention thereis provided a lithographic projection apparatus comprising:

[0048] an illumination system for supplying a projection beam ofradiation;

[0049] a first object table for holding patterning means capable ofpatterning the projection beam according to a desired pattern;

[0050] a second object table for holding a substrate; and

[0051] a projection system for imaging the patterned beam onto a targetportion of the substrate; and

[0052] a balanced positioning system capable of positioning at least onof said object tables in at least two degrees of freedom, thepositioning system comprising:

[0053] at least one balance mass;

[0054] bearing means for movably supporting said balance mass;

[0055] positioning means for positioning said object table in at leastfirst and second degrees of freedom, said first to second degrees offreedom being translations in first and second directions that aresubstantially orthogonal, said positioning means comprising coarse andfine positioning means and being arranged so that reaction forces fromsaid positioning means are channeled to said balance mass; characterizedin that:

[0056] said coarse positioning means comprises a planar electric motorhaving a translator mounted to said object table and a stator extendingparallel to said first and second directions and mounted to said balancemass.

[0057] The forces exerted by the planar motor will be channeled directlyto the balance mass in the first and the second direction as opposed toan H-drive arrangement. In an H-drive arrangement forces may bechanneled indirectly to the balance mass, since the object table isdriven by an X-slider over an X-beam in the X-direction and the X-beamand object table are driven in the Y-direction by two Y-direction linearmotors with corresponding sliders mounted to both ends of the X-beam.Only the beams of the Y-linear motors are mounted to the balance mass.Forces exerted in the X-direction by the X-motor will be channeledindirectly via the X-beam and the Y-direction linear motors to thebalance mass. When a planar motor is used reaction forces in both theX-direction and the Y-direction are directly channeled to the balancemass. Further, with the stator (e.g. a magnet array) mounted to thebalance mass, the mass of the balance mass is desirably increased toreduce its movement range.

[0058] In a vacuum environment it may be advantageous to use the planarmotor also to levitate the object table because it will be difficult touse a gas bearing to levitate the object table in a vacuum environment.The planar motor may also be used to rotate the object table around athird direction being mutually orthogonal to said first and seconddirection.

[0059] The magnate levitation of the planar motor provides for africtionless bearing allowing the balance mass to, freely move in firstand second directions and rotate around the third direction. The balancemass may also be movable in the third direction and/or rotatable aroundone or both of the first and second directions such that it providesbalancing in more than three degrees of freedom. For this purpose thebalance mass may be supported by supports having a low stiffness in thethird direction. The balance mass may be provided with upstanding wallsto raise the center of gravity of the balance mass to the same level inthe third direction as the center of gravity of the object table.

[0060] According to a further aspect of the invention there is provideda method of manufacturing a device using a lithographic projectionapparatus comprising:

[0061] an illumination system for supplying a projection beam ofradiation;

[0062] a first object table for holding patterning means capable ofpatterning the projection beam according to a desired pattern;

[0063] a second object table for holding a substrate; and

[0064] a projection system for imaging the patterned beam onto a targetportion of the substrate; the method comprising the steps of:

[0065] providing a substrate provided with a radiation-sensitive layerto said second object table;

[0066] providing a projection beam of radiation using an illuminationsystem;

[0067] using patterning means to endow the projection beam with apattern in its cross-section;

[0068] projecting the patterned beam of radiation onto target portionsof said substrate;

[0069] wherein during or prior to said projecting step at least one ofsaid object tables is moved in first to third degrees of freedom bycoarse positioning means and in at least a fourth degree of freedom byfine positioning means and, during such movement, reaction forces insaid first to third degrees of freedom are exerted on a balance mass;

[0070] characterized by the further step of:

[0071] channeling reaction forces in said fourth degree of freedom tosaid balance mass.

[0072] In a manufacturing process using a lithographic projectionapparatus according to the invention a pattern (e.g. in a mask) isimaged onto a substrate which is at least partially covered by a layerof radiation-sensitive material (resist). Prior to this imaging step,the substrate may undergo various procedures, such as priming, resistcoating and a soft bake. After exposure, the substrate may be subjectedto other procedures, such as a post-exposure bake (PEB), development, ahard bake and measurement/inspection of the imaged features. This arrayof procedures is used as a basis to pattern an individual layer of adevice, e.g. an IC. Such a patterned layer may then undergo variousprocesses such as etching, ion-implantation (doping), metallization,oxidation, chemo-mechanical polishing, etc., all intended to finish offan individual layer. If several layers are required, then the wholeprocedure or a variant thereof, will have to be repeated for each newlayer. Eventually, an array of devices will be present on the substrate(wafer). These devices are then separated from one another by atechnique such as dicing or sawing, whence the individual devices can bemounted on a carrier, connected to pins, etc. further informationregarding such processes can be obtained, for example, from the book“Microchip Fabrication: A Practical Guide to Semiconductor Processing”,Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN0-07-067250-4.

[0073] Although specific reference may be made in this text to the useof the apparatus according to the invention in the manufacture of ICs,it should be explicitly understood that such an apparatus has many otherpossible applications. For example, it may be employed in themanufacture of integrated optical systems, guidance and detectionpatterns for magnetic domain memories, liquid-crystal display panels,thin-film magnetic heads, etc. The skilled artisan will appreciate that,in the context of such alternative applications, any use of the terms“reticle”, “wafer” or “die” in this text should be considered as beingreplaced by the more general terms “mask”, “substrate” and “target area”or “target portion”, respectively.

[0074] In the present document, the terms illumination radiation andillumination beam are used to encompass all types of electromagneticradiation or particle flux, including, but not limited to, ultravioletradiation (e.g. at a wavelength of 365 nm, 248 nm, 193 nm, 157 nm or 126nm), EUV, X-rays, electrons and ions.

[0075] The invention is described below with reference to an orthogonalreference system based on X, Y and Z-axes. The Z direction may bereferred to as vertical but this should not, unless the context demands,be taken as implying any necessary orientation of the device.

[0076] The present invention will be described below with reference toexemplary embodiments and the accompanying schematic drawings, in which:

[0077]FIG. 1 depicts a lithographic projection apparatus according to afirst embodiment of the invention;

[0078]FIG. 2 is a plan view of a balance mass of the present inventionin the substrate stage of the apparatus of FIG. 1;

[0079]FIG. 3 is a view similar to FIG. 2 but additionally showing thedrive arrangement for the substrate table;

[0080]FIG. 4 is a diagram of the servo system of the balance system ofFIG. 2;

[0081]FIG. 5 is a plan view of the drift control arrangement of a firstvariation of the balance system of the first embodiment;

[0082]FIG. 6 is a plan view of the drift control arrangement of a secondvariation of the balance system of the first embodiment;

[0083]FIG. 7 is an enlarged side view of a driver of the drift controlarrangement of FIG. 6;

[0084]FIG. 8 is a cross-sectional view of the driver along line I-I ofFIG. 7;

[0085]FIG. 9 is a plan view of the drift control arrangement of a thirdvariation of the balance system of the first embodiment;

[0086]FIG. 10 is an enlarged plan view of a driver of the drift controlarrangement of FIG. 9;

[0087]FIG. 11 is a plan view of a fourth variation of the firstembodiment, showing stroke limiters;

[0088]FIG. 12 is a cross-sectional view of a substrate stage of a secondembodiment of the invention;

[0089]FIG. 13 is a cross-sectional view of a substrate stage of a thirdembodiment of the invention; and

[0090]FIG. 14 is a cross-sectional view of a substrate stage of a fourthembodiment of the invention.

[0091]FIG. 15 is a cross-sectional view of a substrate stage of a fifthembodiment of the invention.

[0092] In the drawings, like references indicate like parts.

[0093] Embodiment 1

[0094]FIG. 1 schematically depicts a lithographic projection apparatusaccording to the invention. The apparatus comprises:

[0095] a radiation system LA, IL for supplying a projection beam PB ofradiation (e.g. UV or EUV radiation, x-rays, electrons or ions);

[0096] a first object table (mask table) MT provided with a mask holderfor holding a mask MA (e.g. a reticle), and connected to firstpositioning means for accurately positioning the mask with respect toitem PL;

[0097] a second object table (substrate table) WT provided with asubstrate holder for holding a substrate W (e.g. a resist-coated siliconwafer), and connected to second positioning means for accuratelypositioning the substrate with respect to item PL;

[0098] a projection system (“lens”) PL (e.g. a refractive orcatadioptric system, a mirror group or an array of field deflectors) forimaging an irradiated portion of the mask MA onto a target portion C ofthe substrate W.

[0099] As here depicted, the apparatus is of a transmissive type (i.e.has a transmissive mask). However, in general, it may also be of areflective type, for example.

[0100] The radiation system comprises a source LA (e.g. a Hg lamp,excimer laser, a discharge plasma source, a laser-produced plasmasource, an undulator provided around the path of an electron beam in astorage ring or synchrotron, or an electron or ion beam source) whichproduces a beam of radiation. This beam is passed along various opticalcomponents comprised in the illumination system IL,—e.g. beam shapingoptics Ex, an integrator IN and a condenser CO—so that the resultantbeam PB has a desired form and intensity distribution.

[0101] The beam PB subsequently intercepts the mask MA which is held ina mask holder on a mask table MT. Having passed through the mask MA, thebeam PB passes through the lens PL, which focuses the beam PB onto atarget portion C of the substrate W. With the aid of the interferometricdisplacement measuring means IF and the second positioning means, thesubstrate table WT can be moved accurately, e.g. so as to positiondifferent target portions C in the path of the beam PB. Similarly, thefirst positioning means can be used to accurately position the mask MAwith respect to the path of the beam PB, e.g. after mechanical retrievalof the mask MA from a mask library. In general, movement of the objecttables MT, WT can be realized with the aid of a long stroke module(course positioning) and a short stroke module (fine positioning), whichare not explicitly depicted in FIG. 1.

[0102] The depicted apparatus can be used in two different modes:

[0103] 1. In step mode, the mask table MT is kept essentiallystationary, and an entire mask image is projected in one go (i.e. asingle “flash”) onto a target portion C. The substrate table WT is thenshifted in the X and/or Y directions so that a different target portionC can be irradiated by the beam PB;

[0104] 2. In scan mode, essentially the same scenario applies, exceptthat a given target portion C is not exposed in a single “flash”.Instead, the mask table NIT is movable in a given direction (theso-called “scan direction”, e.g. the Y direction) with a speed v, sothat the projection beam PB is caused to scan over a mask image;concurrently, the substrate table WT is simultaneously moved in the sameor opposite direction at a speed V=Mv, in which M is the magnificationof the lens PL (typically, M=¼ or ⅕). In this manner, a relatively largetarget portion C can be exposed, without having to compromise onresolution.

[0105] The apparatus also includes a base frame BP (also referred to asa base plate or machine frame) to support the components of theapparatus, and a reference frame RF, mechanically isolated from the baseframe BP to support the projection system PL and position sensors suchas the interferometric displacement measuring means IF.

[0106]FIG. 2 shows a balance system according to a first embodiment ofthe invention, which is used in a wafer stage comprising substrate tableWT, of the lithographic apparatus to provide balancing in three degreesof freedom. The arrangement described below may also be used, withsuitable modification, in a reticle stage, comprising mask table MT, ofa lithographic apparatus.

[0107] The balancing system of the first embodiment comprises a balanceframe 2 (balance mass) which is supported by substantially frictionlessbeatings 3 so as to be moveable over a guide surface 4 provided on themachine base frame. The frictionless bearings 3 may be aerostaticbearings or hydrostatic or magnetic bearings, for example.Alternatively, if the required range of movement is relatively small,elastic guiding systems such as flexures or parallel leaf springs can beused. The arrangement may also be reversed—i.e. the bearings provided inthe machine frame and acting against a guide surface on the underside ofthe balance frame. The guide surface 4 is parallel to the XY planedefined for the apparatus and the balance frame 2 is free to translatein the X and Y directions and to rotate (Rz) about axes parallel to theZ direction.

[0108] The positioning system 10, shown in FIG. 3, is placed within orupon the balancing frame 2 and has relatively large ranges of movementin the X and Y directions. It is important that the center of mass ofthe positioning system 10 be as close as possible in the Z direction tothe center of mass of the balance frame 2. In particular, it ispreferred that the vertical separation of the two centers of mass issubstantially less than 100 mm and ideally zero.

[0109] Elastic posts or buffers 5 limit the movement of the balanceframe 2 to prevent it leaving the guide surface 4.

[0110] The positioning system is arranged such that the reaction forcesacting in opposition to the drive forces exerted on the driven objectare transmitted to the balance frame via mechanical or electromagneticconnections. These connections are positioned in or close to the XYplane containing the center of mass of the combined system of balanceframe 2 and positioning system 10. The connections may, for example, beaerostatic bearings with bearing surfaces perpendicular to the XY planeor electromagnetic linear actuators with, for example, magnets attachedto the balance frame 2 and coils or armatures attached to thepositioning system, such that the line of action of the electromagneticforces lies in the same XY plane as the combined center of mass.

[0111]FIG. 3 shows such an arrangement where the positioning system 10is a so-called H-drive. The H-drive 10 comprises an X-beam 11 mounted ator near its ends to respective sliders 12 a, 12 b. Sliders 12 a, 12 bcarry armatures of linear motors that act in concert with elongatemagnet tracks 13 a, 13 b, which are mounted in the long sides 2 a, 2 bof rectangular balance frame 2, to translate X-beam 11 in the Ydirection. The object to be positioned, in this case wafer table WT, isdriven in the XY plane by a further slider 14 which is mounted on X-beam11. Slider 14, similarly to sliders 12 a, 12 b, carries the armature ofa linear motor to act against a magnet track 15 mounted in X-beam 11 totranslate slider 14 along the X-beam and hence position wafer table WTin the X direction. Independent control of the positions of sliders 12a, 12 b allows the angle between X-beam 11 and the balancing frame to bevaried and hence the Rz (rotation about the Z axis) position of thewafer table WT to be controlled within a certain range to compensate forany yaw movements of the balancing frame. It will be appreciated that,for this reason and also due to distortion of the balance frame causedby shear components in the resultant force on the balance frame, the Xand Y directions in which the drivers exert forces will not always beexactly orthogonal. By this arrangement, the reaction forces in the Yand Rz directions are transferred directly to the balance frame 2.Sliders 12 a, 12 b also carry air bearings 16 a, 16 b which act againstupstanding walls 21 a, 21 b provided on the balance frame 2 to transmitreaction forces in the X direction to balance frame 2. Instead of a pairof thrust bearings 16 a, 16 b to transmit the X direction forces, asingle pre-loaded beating or an opposed pad bearing, for example, may beused on one of the two sides and is often preferred as it avoidsdifficulties with cosine foreshortening when the X-beam 11 is notperpendicular to the balance frame 2.

[0112] As illustrated, the positioning system is supported in the Zdirection and against Rx, Ry rotations by the balance frame. Thisfunction can also be performed by the guide surface 4 for the whole or apart of the positioning system (e.g. the wafer table WT), by a separatesurface or surfaces fixed relative to the base frame, or by acombination of the above.

[0113] If so-called planar motors are used, reaction forces in the X andY directions are transmitted to the balance frame via a magnet (or coil)plate in the XY plane. The magnet (or coil) plate may form part of thebalance frame in the XY plane, and so desirably increase its mass toreduce its movement range. Again, the magnet (or coil) plate may besupported in Z, Rx and Ry directions by a second balance mass or byseparate means, such as frictionless bearings, over the machine base.

[0114] The drive forces exerted on the driven object, in this case thewafer table WT, give rise to equal and opposite reaction forces which,according to the invention, are exerted on the balance frame (balancemass). From Newton's laws, it will be seen that the ratio of thedisplacements of the driven object and the balance mass is inverselyproportional to their mass ratio, i.e.: $\begin{matrix}{\frac{x_{1}(t)}{x_{2}(t)} = {- \frac{m_{2}}{m_{1}}}} & \lbrack 1\rbrack\end{matrix}$

[0115] where x_(i) is the displacement of mass i relative to the commoncenter of gravity and m, is the mass of mass i. In this context itshould be noted that the balance mass ratio may vary according to thedirection in which displacement occurs. In the present embodiment, theX-beam 11 and Y-sliders 12 a, 12 b move with the wafer table WT fordisplacements in the Y direction whereas the wafer table moves relativeto the X-beam 11 for displacements in the X-direction. Thus the drivenmass for displacements in the Y-direction is the combined mass of thewafer table WT, X-slider 14, X-beam 11 and Y-sliders 12 a, 12 b. On theother hand, for displacements in the X-direction, the driven mass isonly the mass of the wafer table WT and X-slider 14; the X-beam andY-sliders instead form part of the balance mass. Since the X-beam andY-sliders have a similar mass to the wafer table WT and X-slide 14, thiscan make a significant difference to the balance mass ratio.

[0116] By making the balance frame 5 to 20 times more massive than thecombined moving mass of the positioning system, the motion ranges of thebalance frame can be restrained and the overall footprint of thebalancing system confined as desired.

[0117] If, during positioning, the center of mass of the balance frameis not in line with the center of mass of one of the positioning devicesin the X or Y direction, reaction forces in that direction may cause yawmotion of the balance frame. In some cases, e.g. circular motion of thedriven object around a point offset from the center of mass of thebalance frame, yaw motions can be caused that cumulate over time ratherthan cancel. To prevent excessive yaw motion, a negative feedback servosystem is provided. This control system is also adapted to correctlong-term cumulative translations (drift) of the balance frame thatmight arise from such factors as: cabling to the positioning devices,misalignment of the positioning drives, minute friction in the bearings3, etc. As an alternative to the active drift control system describedbelow, a passive system, e.g. based on low-stiffness springs, may forexample be used.

[0118]FIG. 4 shows the control loop of a servo system 30 as referred toabove. The X, Y and Rz setpoints of the balance mass with respect to themachine frame are supplied to the positive input of subtractor 31, whoseoutput is passed to the servo controller 32. The servo controllercontrols a three-degree-of-freedom actuator system 33, which applies thenecessary corrections to the balance frame 2. Feedback to the negativeinput of subtractor 31 is provided by one or moremultiple-degree-of-freedom measurement systems 34 which measure theposition of the balance frame and driven mass. The positions of bothbalance frame and driven mass may be measured relative to a fixed frameof reference. Alternatively, the position of one, e.g the balance mass,may be measured relative to the reference frame and the position of thedriven mass measured relative to the balance mass. In the latter casethe relative position data can be transformed to absolute position dataeither in software or by hardware.

[0119] The set points of the servo system 30 are determined so as toensure that the combined center of mass of the positioning device(s) andbalance frame 2 remains unchanged in the XY plane. This defines thecondition:

m ₁ . {right arrow over (u₁)}( t)+m ₂. {right arrow over (u₂)}(t)=m ₁ .{right arrow over (u₁)}(0)+m ₂ .{right arrow over (u₂)}(0)  [2]

[0120] where {right arrow over (u_(i))} (t) is the vector displacementof mass i in the X-Y plane at time t relative to a fixed referencepoint. The error signal between the calculated (using equation [2]) andmeasured positions is provided to the actuation system 33, which appliesappropriate correction forces to the balance frame 2. The lowestresonance mode of the balancing frame and/or machine base is at least afactor of five higher than the servo bandwidth of the drift controlsystem.

[0121] To minimize cumulative yaw motions of the balancing frame, thecontrol mode is configured with a low servo bandwidth but a fixedsetpoint (e.g. zero yaw). Similar to a passive (e.g. spring) driftcontrol, the servo bandwidth for yaw serves as a low-pass filter tominimize transient moments on the machine base about the yaw axis. Inother words, only reaction forces to correct long-term (low frequency)movements are transmitted to the base frame.

[0122]FIG. 5 shows a drift control actuation system 33 a according to afirst variation of the first embodiment. This system comprises threeLorentz(force)-type linear motors (e.g. voice coil motors, ironlessmulti-phase linear motors, etc.) 331, 332, 333. Two of these motors 331,332 act in one direction, e.g. the X direction, and are spaced apartwidely in the other, e.g. Y. The third motor 333 acts in the otherdirection, e.g. Y, and through or near to the combined center of mass ofthe balancing frame. The drivers are preferably Lorentz force motorshaving a magnet plate or coil elongate in the direction perpendicular tothe direction in which they act so that they can exert a force in thegiven direction irrespective of the position of the balance frame 2 inthe perpendicular direction.

[0123] The above arrangement, using three drivers, is advantageous asbeing the simplest possible arrangement, but if the balance frame 2 isan open rectangle with limited resistance to shear, four motors may beused, each acting along or close to the neutral axis of one side memberof the frame, thereby to minimize bending of the frame members. Such anarrangement is shown in FIG. 6. Here, four drivers 334 a, 334 b, 334 c,334 d are used—one at each corner, arranged to exert force parallel toand in line with a respective one of the four beams 2 a, 2 b, 2 c, 2 d.Each of the four drivers can be, as before, Lorentz type linear motors.A further alternative is to use two planar motors, each exerting forcesin the X and Y directions, to provide combined control in X, Y andR_(z).

[0124] An alternative form of driver 334 is shown in FIG. 7, which is aside view, and FIG. 8, which is a sectional view along line I-I in FIG.7. Driver 334 consists of a rotary Lorentz motor 335 (such as anironless moving coil motor, a DC or AC brushless motor, etc.) mounted onthe base or machine frame BP and connected to the balance frame 2 by arotary-linear motion transformer 336. The rotary-linear motiontransformer 336 comprises a disc 336 a attached rigidly to the driveshaft 335 a of the motor 335 and having an eccentrically mounted pin 336b. Pin 336 b forms an axle for two wheels 336 c, 336 d which engage acoupling frame 336 e mounted on the balance frame 2. Coupling frame 336e is elongate perpendicular to the direction of action of the force tobe exerted on the balance frame and generally C-shaped in cross-section.It surrounds the wheels 336 c, 336 d so that each engages a respectiveone of opposed bearing surfaces 336 g, 336 f. Bearing surface 336 ffaces towards balance frame 2 and bearing surface 336 g faces away.Thereby, if motor 335 is energized to rotate disc 336 a clockwise inFIG. 8, wheel 336 c will be caused to bear on surface 336 g and exert aleftwards push force on balance frame 2. Similarly anticlockwiserotation of disc 336 a would exert a pull force rightwards on balanceframe 2.

[0125] Rotary-linear motion transformer 336 is arranged to besubstantially friction free and reversible with no play so that driftactuation control can be carried out in a force mode rather than aposition mode. The position of balance frame 2 can additionally bemeasured via rotary encoders (not shown) provided on disc 336 a.

[0126] A further alternative drift control system is illustrated in FIG.9, which is a plan view of the balance frame 2, and FIG. 10, which is anenlarged view of one of the drive mechanisms 337 used in thisalternative. Drive mechanism 337 is a so-called “double scara mechanism”which consists of two crank-con'rod mechanisms connected to a commonpivot point. Each crank-control mechanism consists of a crank 337 adriven by a Lorentz-type torque motor 337 b and a con'rod 337 cconnecting the end of crank 337 a to common pivot point 337 d. Torquemotor 337 b is mounted on the base frame BP and its drive shaft fixedagainst translation so that reaction forces are transferred to the baseframe BP.

[0127] The drift actuation system of FIGS. 9 and 10 is over-determinedsince only three drives are sufficient to control the balance frame inthree degrees of freedom but the additional motor provides the samebenefits as that of the arrangement of FIG. 6.

[0128] The position and orientation of the balance frame 2 can bedetermined from the crank angles, which may be measured by rotaryencoders provided on the drive shafts of motors 337 b. In the servocontrol system, two coordinate transforms are provided: one to convertinformation of the angular position of the cranks 337 a to X, Y, Rzcoordinates of the position of the balance mass 2; and one to convertthe forces determined by the controller 33 into torques for the drivemotors 337 b.

[0129] As mentioned, the above-described drift control arrangements mayinclude linear or rotary position sensors incorporated into the linearor rotary drive mechanisms. Alternatively an independent positionmeasuring system, e.g. a grid encoder or a 2-dimensional positionsensing detector, may be employed. Such a system may have multipleoutputs which can be transformed into X, Y and Rz coordinates or mayprovide independent measurement of the XY positions of two points on thebalancing frame, preferably diagonally opposite corners. Such apositioning mechanism may measure the position of the balance frame 2relative to the base frame or, in ultra-precision machines, to avibration-isolated metrology frame.

[0130] To prevent the balance frame 2 from drifting out of range, e.g.in the event of an error situation, a stroke limiting device may beprovided between the balancing frame and the base frame. An example ofsuch a device is shown in FIG. 11 which is a view showing across-section through the lower part of the balancing frame 2. In thisdevice, three pins 40 project upwardly from the bearing surface 4 of thebase frame BP and engage open-ended slots 41 in the balance frame 2. Theslots 41 and pins 40 are sized and arranged to confine movement of thebalance frame 2 to a predefined envelope in X, Y and Rz. The pins 40 maybe resilient or spring-loaded to cushion any shock to the balance frame2 in the event of a crash. The stroke limiting device may bekinematically inverted, with pins projecting from the balance frame 2engaging in slots on the base frame BP.

[0131] If it is not possible to arrange that the centers of mass of thepositioning devices and the balance frame 2, as well as the drivingforces of the various actuators, lie in the same XY plane, drivingforces acting at the offset will cause tilting moments Tx, Ty, i.e.moments tending to rotate the balance frame 2 and the positioningdevices around the X and Y axes. If the balance frame 2 is supported inthe Z, Rx and Ry directions with relatively high stiffness, tiltingmoments Tx, Ty will be transmitted to the base frame BP and causevibrations there. Also, although coarse positioning is usually onlyperformed in the X, Y and Rz directions, fine positioning actuatorsincluded in substrate table WT for the moveable object are commonlycapable of positioning in all six degrees of freedom. The reactionforces from motions of the fine positioning system in Z, Ry, Rx, as wellas the other degrees of freedom, can also cause vibrations iftransmitted to the base frame BP.

[0132] Accordingly, the balance frame 2 is supported in the Z, Rx and Rydirections with low stiffness supports, comprised in bearings 3. Suchsupports may be low-stiffness frictionless bearings or elastic or gassprings in combination with frictionless bearings. Large-gap airbearings may also be used. As with the use of passive components tocontrol drift in X, Y and Rz directions, the spring constants are chosenso that the Eigen-frequency of the balance frame mass-spring system issubstantially lower, e.g. by a factor of 5 to 10, than the lowestfundamental frequency of the motion of the positioning devices. Shouldthe wafer table WT be supported in Z, Rx, Ry by the guiding surface 4 onthe base frame rather than on the balance frame, the base frame memberproviding guiding surface 4 can be treated as a second balance mass forZ, Rx and Ry and be passively supported as described.

[0133] Embodiment 2

[0134] The substrate stage, comprising substrate table WT, of a secondembodiment of the invention, which may be the same as the firstembodiment save as described below, is shown in FIG. 12.

[0135] In the second embodiment, the balance mass 406 takes the form ofan open box with a flat interior base 407, forming a guide surface forthe wafer table WT, and upstanding side walls 408 serving to raise thecenter of gravity of the balance mass 406. The substrate table WTincludes a fine positioning mechanism 417 operating in 6 degrees offreedom for the substrate W and a so-called air-foot forming asubstantially frictionless bearing allowing the substrate table WT to bemoved over the guide surface 407.

[0136] Movement of the substrate table WT is effected by the coarsepositioning mechanism. This includes X-beam 415 relative to which thesubstrate table WT is driven by an X-driver (not shown) which has at itsends sliders 411 which include the translators of Y-direction linearmotors to drive the X-beam, and hence the substrate table WT, in the Ydirection and by applying different forces to the opposite ends of theX-beam in R_(z). The stators 409 of the Y-direction linear motors areprovided in shoulders of the balance mass 406. Y-direction and R_(z)reaction forces from movement of the substrate table are thus directlyapplied to the balance mass 406. X-direction reaction forces aretransferred to the balance mass 406 via bearings between the slides 411and the sidewalls 408 of the balance mass 406.

[0137] Because the substrate table WT is guided over the base 407 of thebalance mass 406, Z, Ry and Rx reaction forces from the correspondingmovements of the substrate WT by the fine positioning mechanism 417, arealso transmitted directly to the balance mass 406. Any tilting movementsTx, Ty arising from imperfect adjustment of the centers of gravity ofthe substrate table WT and balance mass 406 as well as the lines offorce exerted by the X- and Y-drives, are also transmitted to thebalance mass 406 via the air-pot 419 and the stiffness of the Y-linearmotors.

[0138] To enable the balance mass 406 to absorb the reaction forces inall six degrees of freedom it must be free to move in all six degrees offreedom. This is achieved by supporting it from the base frame BP by aplurality of supports 403, which have a low stiffness in the Zdirection, and substantially frictionless bearings 405, which bear onthe lower surface of balance mass 405. The lower surface of balance mass406 is flat, or has flat regions of sufficient size to accommodate themaximum expected or allowed range of movement of balance mass 406. Sincethe balance mass 406 is much, e.g. 5 to 10 times, heavier than thesubstrate table WT, the range of movement of the balance mass 406 willbe much less than the range of movement of the substrate table WT.

[0139] Embodiment 3

[0140]FIG. 13 depicts the substrate stage, comprising substrate tableWT, of a third embodiment of the invention, which may be the same as thefirst or second embodiments described above.

[0141] In the third embodiment, the balance mass is divided into twoparts 506, 507. The first balance mass part 506 comprises a rectangularframe surrounding the substrate table WT. Opposite sides of the firstbalance mass part 506 have mounted thereon the stator, e.g. the magnettrack, of the Y-direction linear motors. The translators, e.g. coils, ofthe Y-direction linear motors, are mounted in sliders 511 at the ends ofX-beam 515. The X-beam includes the stator of X-linear motor and thetranslator is mounted to the substrate table WT. Y- and R_(z)-reactionforces from the Y linear motor, which forms the coarse positioningmechanism together with the X-linear motor, arc transmitted directly tothe first balance mass part 506 and the X-reaction forces aretransmitted via thrust bearings (not shown). To absorb the X- andY-reaction forces, the first balance mass part 506 is supported bysubstantially frictionless bearings, eg. air bearings, 505 allowing itto move in X, Y and R_(z).

[0142] Second balance mass part 507 takes the form of a plate and isdisposed underneath the substrate table WT. Its upper surface 508 isflat and forms a guide surface over which the substrate table WT isborne by air foot 519. In this way, any reaction forces in Z, R_(x) andR_(y) from movements of the wafer W by fine positioning mechanism 517are transmitted to second balance mass part 507, which is supported fromthe base frame BP by a plurality of supports 503 having a low stiffnessin the Z-direction. These supports may be, for example, mechanical orgas springs.

[0143] Embodiment 4

[0144] A fourth embodiment of the invention is a modification of thesecond embodiment for use in a vacuum. The substrate stage, includingsubstrate table WT, is shown in FIG. 14.

[0145] As in the second embodiment, the balance 606 takes the form of anopen box. In this embodiment, the base of the box includes the stator627, e.g. magnet array, of a planar motor whose translator 635 ismounted to the wafer table WT. More information on a planar motor can begleaned from U.S. Pat. No. 5,886,432, which is incorporated herein byreference. As before, upstanding walls 625 serve to raise the center ofgravity of the balance mass 606 to the same horizontal plane as that ofthe substrate table WT. The planar motor 627, 635 may be arranged tolevitate as well as translate the substrate table or additional bearingscan be provided. Reaction forces from the X, Y and possibly, Rztranslations of the planar motor are channeled to the balance mass 606.

[0146] Embodiment 5

[0147] A fifth embodiment of the invention is a modification of thefourth embodiment as shown in FIG. 15. As in the fourth embodiment thesubstrate table WT is movable in the plane of the stator 627 of theplanar motor (i.e. the X and Y direction). Reaction forces from the X, Yand R_(z) movements of the coarse positioning mechanism planar motor)are transmitted directly to the balance mass 606. Reaction forces in alldegrees of freedom for the fine position mechanism 617 are transmittedthrough the stiffness of the planar motor or additional bearingsprovided for the substrate table to the balance mass 606. The balancemass is mounted on bearings 605 and low-stiffness supports 603 in thesame way as the second embodiment.

[0148] Whilst we have described above specific embodiments of theinvention, it will be appreciated that the invention may be practicedotherwise than described. The description is not intended to limit theinvention. In particular it will be appreciated that the invention maybe used in the reticle or mask stage of a lithographic apparatus and inany other type of apparatus where fast and accurate positioning of anobject in a plane is desirable.

1. A lithographic projection apparatus comprising: an illumination system for supplying a projection beam of radiation; a first object table for holding patterning means capable of patterning the projection beam according to a desired pattern; a second object table for holding a substrate; and a projection system for imaging the patterned beam onto a target portion of the substrate; and a balanced positioning system capable of positioning at least one of said object tables in more than three degrees of freedom, the positioning system comprising: at least one balance mass; bearing means for movably supporting said balance mass; coarse positioning means for positioning said object table in first to third degrees of freedom, said three degrees of freedom being translation in first and second directions and rotation about a third direction, said first, second and third directions being substantially mutually orthogonal; and fine positioning means for positioning said object table in at least a fourth degree of freedom substantially orthogonal to said first, second and third degrees of freedom, said coarse and fine positioning means being arranged so that reaction forces from said coarse and fine positioning means are channeled to said balance mass; characterized in that: said balance mass is supported by said bearing means so as to be substantially free to move in at least said fourth degree of freedom.
 2. Apparatus according to claim 1 further comprising a base plate having a guide surface extending parallel to said first and second directions and wherein said bearing means comprises a plurality of substantially frictionless bearings bearing on said guide surface and connected to said balance mass by supports having a low stiffness in said third direction.
 3. Apparatus according to claim 2 wherein said bearings are selected from the group comprising aerostatic bearings, hydrostatic bearings and magnetic bearings.
 4. Apparatus according to claim 2 or 3 wherein said supports are selected from the group comprising elastic springs and gas springs.
 5. Apparatus according to claim 1 further comprising a base plate having a guide surface extending parallel to said first and second directions and wherein said bearing means comprises a plurality of low-stiffness substantially frictionless bearings on said guide surface.
 6. Apparatus according to any one of claims 2 to 5 wherein said balance mass comprises a generally rectangular frame having its sides generally parallel to said first and second direction.
 7. Apparatus according to claim 6 wherein said generally rectangular frame has a central opening and wherein said object table is at least partly disposed in said central opening.
 8. Apparatus according to claim 7 wherein said object table has a low stiffness bearing for supporting said object table over said guide surface.
 9. Apparatus according to claim 1 wherein said balance mass has a guide surface extending parallel to said first and second directions and said apparatus further comprises a base having substantially frictionless bearings bearing against said guide surface and supported from said base by supports having a low stiffness in said third direction.
 10. Apparatus according to claim 9 wherein said balance mass has a further guide surface substantially parallel to said guide surface and said object table is provided with a substantially frictionless bearing bearing on said further guide surface.
 11. Apparatus according to claim 10 wherein said coarse positioning means comprises a beam extending generally parallel to said first direction and a first drive means for driving said object table relative to said beam in said first direction and second and third drive means connected to respective ends of said beam for driving said beam relative to said balance mass in said second direction.
 12. Apparatus according to claim 10 wherein said coarse positioning means comprises a planar electric motor having a stator mounted to said further guide surface of said balance mass and a translator mounted to said object table.
 13. Apparatus according to claim 1 wherein said balance mass comprises first and second balance mass parts, said first balance mass part being moveable in said first to third degrees of freedom and said second balance mass part being moveable in said fourth degree of freedom.
 14. Apparatus according to claim 13 wherein: said apparatus comprises a base; said first balance mass part comprises a generally rectangular frame surrounding said object table, said first balance mass part being supported from said base by substantially frictionless bearings and said coarse positioning means acting between said first balance mass part and said object table; and said second balance mass part has a guide surface extending substantially parallel to said first and second directions and is supported from said base by supports having a low stiffness in said third direction, said object table being supported over said guide surface by a substantially frictionless bearing.
 15. A lithographic projection apparatus comprising: an illumination system for supplying a projection beam of radiation; a first object table for holding patterning means capable of patterning the projection beam according to a desired pattern; a second object table for holding a substrate; and a projection system for imaging the patterned beam onto a target portion of the substrate; and a balanced positioning system capable of positioning at least on of said object tables in at least three degrees of freedom, the positioning system comprising: at least one balance mass; bearing means for supporting said balance mass so as to be substantially free to move in said three degrees of freedom; and driving means for acting directly between said object table and said balance mass to position said object table in said three degrees of freedom; characterized in that: said balance mass comprises a generally rectangular frame having its sides generally parallel to said first and second directions, and a central opening in which said object table is at least partly disposed.
 16. A lithographic projection apparatus comprising: an illumination system for supplying a projection beam of radiation; a first object table for holding patterning means capable of patterning the projection beam according to a desired pattern; a second object table for holding a substrate; and a projection system for imaging the patterned beam onto a target portion of the substrate; and a balanced positioning system capable of positioning at least on of said object tables in at least two degrees of freedom, the positioning system comprising: at least one balance mass; bearing means for movably supporting said balance mass; positioning means for positioning said object table in at least first and second degrees of freedom, said first to second degrees of freedom being translations in first and second directions that are substantially orthogonal, said positioning means comprising coarse and fine positioning means and being arranged so that reaction forces from said positioning means are channeled to said balance mass; characterized in that: said coarse positioning means comprises a planar electric motor having a translator mounted to said object table and a stator extending parallel to said first and second directions and mounted to said balance mass.
 17. Apparatus according to any one of the preceding claims having more than one first object table and/or more than one second object table wherein reaction forces to drive forces for positioning a plurality of object tables are directed to a common balance mass or masses.
 18. A method of manufacturing a device using a lithographic projection apparatus comprising: an illumination system for supplying a projection beam of radiation; a first object table for holding patterning means capable of patterning the projection beam according to a desired pattern; a second object table for holding a substrate; and a projection system for imaging the patterned beam onto a target portion of the substrate; the method comprising the steps of: providing a substrate provided with a radiation-sensitive layer to said second object table; providing a projection beam of radiation using an illumination system; using patterning means to endow the projection beam with a pattern in its cross-section; projecting the patterned beam of radiation onto target portions of said substrate; wherein during or prior to said projecting step at least one of said object tables is moved in first to third degrees of freedom by coarse positioning means and in at least a fourth degree of freedom by fine positioning means and, during such movement, reaction forces in said first to third degrees of freedom are exerted on a balance mass; characterized by the further step of: channeling reaction forces in said fourth degree of freedom to said balance mass.
 19. A device manufactured according to the method of claim 18 . 